feat(vpn): VPN on-demand Tailscale + agent studioE5 standalone

- Agent studioE5 standalone en Go (console + systray)
- VPN on-demand via tailscaled + tailscale up (authkey Headscale)
- Resolver/serveur dans le tailnet studioe5
- Caddy on-demand TLS pour les instances
- Nouveaux endpoints serveur /api/internal/send-to-node
- Suppression des anciens binaires edubox-agent
- Suivi dans SUIVI_VPN_ONDEMAND.md

server/app/dashboard/download/page.tsx

@@ -1,6 +1,7 @@
 import { Card, CardHeader, CardTitle, CardContent } from "@/components/ui/card";
 
 const AGENT_VERSION = "0.3.0";
+const AGENT_BIN_NAME = "studioE5-agent";
 
 export const dynamic = "force-dynamic";
 
@@ -15,8 +16,8 @@ export default function DownloadPage() {
             <CardTitle>Windows</CardTitle>
           </CardHeader>
           <CardContent>
-            <p className="text-sm text-muted-foreground mb-4">Agent EduBox pour Windows (64 bits)</p>
-            <a href={`/edubox-agent-v${AGENT_VERSION}.exe`} download className="inline-flex items-center justify-center rounded-md text-sm font-medium bg-primary text-primary-foreground hover:bg-primary/90 h-10 px-4 py-2 w-full">Télécharger (.exe)</a>
+            <p className="text-sm text-muted-foreground mb-4">Agent studioE5 pour Windows (64 bits)</p>
+            <a href={`/${AGENT_BIN_NAME}-v${AGENT_VERSION}.exe`} download className="inline-flex items-center justify-center rounded-md text-sm font-medium bg-primary text-primary-foreground hover:bg-primary/90 h-10 px-4 py-2 w-full">Télécharger (.exe)</a>
           </CardContent>
         </Card>
       </div>